RE's calculator is just fine if your not overly concerned (anal ) about the enclosure design as to what the port size is vs port tuning.
RE's calculator make's the port longer to adjust for the end correction factor ( Half the centerline width of the port is added to the end of the port ) and if you compare RE's calculations to any other enclosure design calculator, the results will not be the same.
An contrary to PV statement " None of the online calculators are accurate in the least. "
is not that none of them are accurate, they only only accurate to the degree of which what information is required ( input ) to calculate the end result of the design in which it code was written to perform it.
So, regardless of what calculator is used, there are going to be discrepancies from one to the next, comparing one to the other is a waste of time, just use one that you want use go for it, the minor differences are not going to amount to squat for your a daily banger.
And if your calculating for an SPL/ Comp enclosure, that won't matter either because the first time " meter it " given what you think it will hit at ( dB and freq wise ) based on what you " designed it at " rarely is the case it will.
Phil
Woodlawn Cabinetry